1. Background of the Invention
The invented apparatus and methods pertain to acoustic inspection of an object such as, but not limited to, a part for an aircraft or other vehicle, to determine whether an object has incurred damage under an applied load, as well as the location and extent of any such damage. The invented apparatus and method can be used in the field of destructive or non-destructive stress testing of an object to determine whether the object is suitable for use in a particular application, for example. In addition, the invented apparatus and method can distinguish and categorize different types of damage to an object, as well as their relationship to the degree and manner in which a load is applied to the object.
2. Description of the Related Art
Acoustic emission (AE) testing uses an acoustic sensor to detect sound emitted by the occurrence of damage in an object subjected to an applied load. The sensor""s output is analyzed to distinguish acoustic signals associated with background noise from acoustic signals related to damage of the object, such as sound emitted by the breaking of the object""s materials under the applied load. By monitoring the acoustic signals emitted by the object under the applied load, much information can be obtained regarding the object""s structural integrity and capability to withstand stresses in a particular application. However, AE testing is purely passive in nature, and because acoustic events may be indistinguishable as resulting from noise or damage, AE systems are incapable in some circumstances of capturing valuable or essential information regarding the true condition of the object.
Acousto-ultrasound (AU) testing uses a transducer to induce acoustic signals in an object as well as a sensor to detect the induced acoustic signals. After traveling through the object, the induced acoustic signals are detected by one or more sensors and are used to determine whether the object has incurred damage under an applied load. If an object has incurred damage, the induced acoustic signals will be disturbed in a manner which permits detection of the damage. AU testing provides a significant advantage in that it permits an object to be probed for the presence of damage. However, in applications in which it is desired to detect damage in an object that may occur at some indefinite time in the future, the AU system""s power supply (e.g., a battery) may become exhausted through continuous use, or the AU system""s components (e.g., the transducer or sensor) may wear out before the end of the object""s useful life. It would be desirable to provide an apparatus and methods which can effectively detect damage in an object although such damage may occur at some indefinite time in the future.
Another problem related to this invention pertains to the detection of different categories of damage that can occur in an object. For example, composite materials are made of two or more materials that are integrated together, and they can be damaged under an applied load by failure of any one or some combination of its materials. It would be desirable to distinguish different categories of damage occurring in an object composed of composite or non- homogeneous materials, for example, to provide greater understanding of how the object is damaged under an applied load so that the object can configured and used in a relatively advantageous manner for a particular application.
Another problem related to this invention is that the acoustic behavior of an object changes as the object undergoes damage, so the sounds emitted by the occurrence of the same class of damage will change depending upon the amount of damage incurred by the object. The relationship between acoustic behavior and the amount of damage incurred is very difficult to ascertain and program into an AE or AU system, if it can be determined at all. It would be desirable to provide an apparatus and method in which acoustic signals resulting from damage can be readily determined without the need to determine relatively complex, non-linear relationships between such signals and the amount of damage incurred by the object.
The invented apparatus and methods have as their objects to overcome the above-stated problems with previous devices and techniques, and do in fact overcome such problems and provide significant advantages over the prior art.
The invented apparatus has a passive mode of operation in which the apparatus xe2x80x9clistensxe2x80x9d to sound emitted by the object to detect an acoustic signal that could possibly be the result of damage. Upon detecting such an acoustic signal, the apparatus switches to an active mode to probe the object with an induced acoustic signal to determine whether the sensed acoustic signal was the result of actual damage or is instead not the result of any damage. After making this determination, the apparatus can return to the passive mode to permit continuous monitoring of the object. Because the apparatus is normally in a passive mode of operation, it does not overuse its elements or prematurely exhaust its power supply as do previous apparatuses of this nature. In addition, the ability of the apparatus to actively probe the object permits the apparatus to accurately distinguish the sound of object damage from ambient noise.
The apparatus can include at least one sensor, a mapping unit, a controller, and at least one transducer. The sensor is mounted to sense an acoustic signal from the object, and generates a signal indicative of such signal. The mapping unit is coupled to receive the sensed signal from the sensor, and generates damage data based on the sensed signal. In the passive mode of operation, the mapping unit generates the damage data to indicate whether or not the object has incurred possible damage or conversely no damage, based on the sensed signal. The controller is coupled to receive the damage data from the mapping unit. If the damage data indicates that the object has incurred no damage, the controller maintains the apparatus in the passive mode. If the damage data indicates possible damage to the object, the controller switches the apparatus from the passive mode to the active mode by generating and outputting induced signal data. The transducer is mounted to the object and coupled to the controller, and induces acoustic signal in the object, based on the induced signal data. In the active mode, the induced acoustic signal is sensed by the sensor after traveling through at least the portion of the object that has incurred possible damage. The sensor generates the sensed signal that is indicative of the induced acoustic signal received by the sensor, and outputs the sensed signal to the mapping unit. In the active mode, the mapping unit generates damage data indicative of whether the object has incurred actual damage, or conversely, no damage, based on the sensed signal. The controller receives the damage data indicative of whether the object has incurred actual damage, and thereafter switches the apparatus from active mode to passive mode.
The apparatus can include a display unit coupled to the controller, that the controller uses to generate a display for a user, based on the damage data. The apparatus can also include a memory in which the controller stores the damage data for later reference or to provide a record of the damage incurred by the object under an applied load, for example. The apparatus can also include an alarm unit coupled to the controller, and the controller can generate an alarm signal, based on the damage data, to indicate to a user that the object has been damaged or that the object has been damaged beyond a predetermined limit. The mapping unit can also be such as to classify damage data in different categories. For example, if the object is a composite material, the mapping unit can classify the damage as fiber damage, fiber-matrix interface damage, fiber-matrix debonding, matrix damage such as cracking, fatigue or failure, or may even classify damage to particular localized portions or layers of the object.
A first method in accordance with this invention includes sensing at least one acoustic signal occurring in an object under an applied load, and determining whether the object has incurred possible damage under the applied load, based on the acoustic signal sensed in the sensing step. The method can also include inducing at least one acoustic signal in the object if the determining step indicates that the object has incurred possible damage. The method can also include sensing the induced acoustic signal from the object, and a step of determining whether the possible damage is actual damage of the object, based on the induced acoustic signal sensed in the later sensing step.
A second method in accordance with this invention includes sensing at least one acoustic signal in an object subjected to an applied load to generate a sensed signal, extracting at least one feature from the sensed signal, and classifying the sensed signal as pertaining to one of a plurality of categories including at least damage and no damage, based on the feature.
These together with other features and advantages, which will become subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being made to the accompanying drawings, forming a part hereof wherein like numerals refer to like parts throughout the several views.